A semiconductor IC (Integrated Circuit) device includes a fine trench structure. A typical example of the fine trench structure is a STI (Shallow Trench Isolation) structure. The STI structure is a device isolation region that isolates active regions of a semiconductor device from each other. The STI structure is formed by forming a fine trench in a silicon substrate and filling the fine trench with a dielectric material.
In the related art, a SOD (Spin-On Dielectric) material is known as an example of the dielectric material that fills the fine trench. In particular, attention is concentrated on an inorganic polymer containing a PHPS (PerHydroPolySilazane: SiH2NH) as the principal ingredient. For example, the PHPS changes into a silicon oxide (SiO2) when being burned in a water-vapor atmosphere. A reaction formula thereof is as follows.SiH2NH+2H2O→SiO2+NH3+2H2 
However, the PHPS shrinks when changing into a silicon oxide. Accordingly, voids occur in the fine trench.
Thus, there is known a technique in which a shrinkage amount of the PHPS is first estimated, an expandable film is pre-formed, and then, the PHPS is filled in the fine trench. The expandable film is a silicon (Si) film. According to the technique, the silicon film is changed into a silicon oxide film and expanded, thereby compensating for the shrinkage of the PHPS and suppressing the occurrence of voids in the fine trench.
In the related art, there is also known a process of changing the silicon film into a silicon oxide film, that is, an oxidation process. According to the process, before the forming of the silicon film, an oxidation barrier film, which blocks the passage of oxygen, is formed in the fine trench. This is to prevent the oxidation from reaching the silicon substrate. The oxidation barrier film is a silicon nitride (Si3N4) film.
However, as the trench becomes finer and finer, it is expected that it will be difficult or impossible to form the oxidation barrier film in addition to the expandable film in the trench.
Also, the oxidation barrier film is left in the trench. If a dielectric material of the oxidation barrier film is different from the dielectric material filling the trench, a band gap difference occurs therebetween. That is, a charge trapping structure is formed in the trench. If charges are trapped in the trench that is a device isolation region, it may obstruct the operation of the semiconductor IC device.